Chemical vapor deposition (CVD) is a well known coating process for producing semi-conducting electronic devices and modifying the surface properties, e.g. abrasive and erosive wear resistance, of a physical object (substrate). For many applications it is crucial that coatings, such as polycrystalline diamond films, be extremely uniform in terms of thickness, morphology, and properties. Inability to produce a sufficiently uniform coating in a practical reactor system can limit the usefulness of many otherwise promising CVD processes.
Hot filament chemical vapor deposition (HFCVD) has been extensively used by researchers to deposit polycrystalline diamond on a variety of substrates. The HFCVD techniques and/or reactor designs are, however, limited in their commercial usefulness due to the inability to produce a diamond coating on multiple pieces with sufficiently uniform thickness, morphology and properties.
The reactor designs typically used for HFCVD are described in detail by Singh, et al. of David Sarnoff Research Center, subsidiary of SRI International, in a research report entitled "Growth of Polycrystalline Diamond Particles and Films By Hot-Filament Chemical Vapor Deposition", and by S. Matsumoto, et al., in a research paper entitled, "Growth of Diamond Particles From Methane-Hydrogen Gas" published in J. Material Science 17, 3106 (1982). A schematic representation of a typical HFCVD reactor used in the above research report and paper is shown in FIG. 1A.
The noted reactor generally comprises a heat proof cylindrical wall tube 1, an end cap 2, a gas inlet tube 3, and a disk shaped base 4. A substrate 5 to be coated, is supported by a substrate holder 6 which rests on and is heated by a substrate heater 7. Substrate 5, substrate holder 6, and the substrate heater 7 are supported within the reactor by two rods 8 which are connected to base 4 by means not shown. Substrate heater 7 is provided with an electrical lead, not shown, which conducts an electrical heating current from a suitable source to the heater. Substrate heater 7 is also provided with a thermocouple 9, by which the temperature of substrate 5 may be measured and transmitted to an external indicating device by means of an electrical lead, not shown.
The reactant gas is introduced into the reactor chamber through gas inlet tube 3 and directed over a spiral filament 10. Filament 10 is supported within the reactor by two rods 11 which are removably secured to reactor cap 2. Filament 10 is also provided with an electrical lead, not shown, to which an electrical current is conducted from a suitable source.
Filament 10 is typically made of .about.0.1 mm diameter wire. It is made from a high melting-point refractory metal, such as tungsten or tantalum. The filament 10 is generally heated to between 1800.degree.-2300.degree. C. by either an AC or DC power supply to dissociate the feed gas, containing a mixture of hydrogen and hydrocarbon, into precursors responsible for diamond formation. The precursors condense at the surface of substrate 5, placed on substrate holder 6 approximately .about.2 to 20 mm from filament 10, to deposit polycrystalline diamond. The temperature of substrate 5 is generally maintained in the range of 700.degree. to 1,000.degree. C. by the radiation heat from filament 10, substrate heater 7, or by a combination thereof.
The typical HFCVD reactor design, as described above, is limited to research purposes where polycrystalline diamond is deposited on a small area (.about.1 cm.sup.2) and on one piece at a time. It is not suitable for depositing a uniform polycrystalline diamond film on multiple small pieces simultaneously.
Numerous researchers have attempted to solve the coating uniformity problem by utilizing many different HFCVD reactor designs. Specific details vary, but most have common features. Illustrative is the improved version of an HFCVD reactor design disclosed by Carl E. Spear in a review paper entitled "Diamond-Ceramic Coating of the Future" published in J. of Am. Ceram. Soc. 72 (2), 171-91 (1989). A schematic representation of this improved reactor is shown in FIG. 2.
The noted reactor generally comprises a heat proof cylindrical wall tube 12, an end cap 13, a gas inlet tube 14, and a disk shaped base 15. A substrate 16 to be coated is supported by a substrate holder/heater 17. Substrate 16 and substrate holder/heater 17 are supported within the reactor by rods 19 which are connected to reactor base 15. Substrate holder/heater 17 is provided with an electrical lead, not shown, which conducts an electrical heating current from a suitable source to the heater. Substrate holder 17 is also provided with a thermocouple 18 by which the temperature of substrate 16 can be measured and transmitted to an external indicating device by means of an electrical lead, not shown.
With the noted reactor, reactant gas is introduced into the reaction chamber through a gas diffuser 20 which is connected to gas inlet tube 14. As the gas is introduced into the reaction chamber, it is directed over a spiral refractory metal filament 21. Spiral filament 21 is supported within the reaction chamber by rods 22 which are connected to reactor cap 13. Filament 21 is also provided with an electrical lead, not shown, to which an electrical current is conducted from a suitable source.
As the reactant gas is introduced into the reaction chamber through gas diffuser 20, it is directed over the heated filament 21. Base 15 is provided with a gas outlet tube 23 for extracting the gas, as well as a gas feed line 24 for monitoring gas pressure within the reactor chamber.
Generally, the axial velocity of reactant gas from a central gas inlet tube 3, as shown in FIG. 1A, will induce a sharp temperature gradient in filament 10 at or near the point where gas impinges on the filament, resulting in its deformation (see FIG. 1B). The filament deformation changes the gap 65 between filament 10 and substrate 5, resulting in non-uniform deposition of polycrystalline diamond film. Gas diffuser 20 used in the noted design (see FIG. 2) helps alleviate to some extent the problems associated with filament deformation. The reactor design is not, however, suitable for depositing a uniform polycrystalline diamond film on multiple small pieces simultaneously.
Further examples of prior art techniques for depositing diamond by HFCVD are as follows: Japanese Patent Application No. 61-302,131, filed Dec. 17, 1986, describes a pre-treatment technique (carburization) for stabilizing the filament surface and prolonging its life against hydrogen embrittlement and water etching. The method, however, suffers from the same drawbacks as described above. It, too, is not suitable for depositing a uniform diamond film on multiple small pieces simultaneously.
U.S. Pat. Nos. 4,707,384 and 4,734,339 describe methods for producing a composite body with one or more polycrystalline diamond layers by HFCVD. However, the methods are directed to using a conventional HFCVD reactor as described above and suffer from similar drawbacks.
Japanese Kokai Patent No. 63-166,797, published Jul. 9, 1988, describes a method of synthesizing diamond using 0.1 mm dia. filament made of an alloy of Ta and Zr and/or Hf such that the proportion of Ta in the alloy is &gt;60% and is &lt;99%. This application does not address the deposition of polycrystalline diamond on multiple small pieces simultaneously.
Japanese Kokai Patent No. 63-159,292, published Jul. 2, 1988, describes a process for depositing diamond film on a substrate with a large surface area and on a curved surface. The process describes using a high-melting point filament in conjunction with a high-melting point metal mesh placed between the substrate and the filament, and applying a positive bias potential with respect to the filament through the metal mesh to deposit diamond on large surface areas. Both the filament and the wire mesh are heated electrically. This process is strictly an electron bias assisted HFCVD and is substantially different from a conventional HFCVD process and the present invention.
European Patent Application Nos. 254,312 and 254,560 disclose an apparatus and methods of depositing polycrystalline diamond using a DC-bias HFCVD method. The method has been claimed to be suitable for depositing diamond coating on large areas. The method, however, does not describe the use of conventional HFCVD to deposit a diamond coating uniformly on multiple small pieces simultaneously.
From the foregoing, it will be seen by those skilled in the art that prior art conventional HFCVD techniques (without the use of electrical bias techniques) are limited to depositing uniform polycrystalline diamond on small areas and on one piece at a time.
It is also recognized by those skilled in the art that a need exists for an improved HFCVD reactor which, in combination with appropriate processing conditions, effectively controls the gas pressure and temperature gradients, and gas flow within the reactor in such a way as to produce uniform polycrystalline diamond coatings on multiple small substrates simultaneously.